Abstract <p>Mechanical activation (MA) of natural aragonite and calcite was performed to study the effect of fluid additives (1–10% aqueous solutions of dimethyl sulfoxide, DMSO, with 1–20 wt % in the MA batch) on the phase composition and particle sizes of the MA products. It was found that the formation of the final products occurs via the stages of dissolution and reprecipitation of CaCO₃ polymorph nanocrystals with sizes of ~20 nm from the fluid phase. During MA of aragonite, the calcite content increases and becomes comparable to that of aragonite. During MA of calcite, aragonite synthesis occurs only during the initial period of MA, and the final product is represented only by calcite. The correspondence of all samples to the standards was studied by powder and high-temperature X-ray diffraction, and the temperature dependences of the unit cell parameters of aragonite and calcite were measured. Thermal analysis of initial and MA samples of aragonite and calcite was performed with description of the specific features of the TG/DSC curves for MA aragonite. Using ultrasound treatment of MA samples with subsequent measurement of particle sizes by light scattering, the property of DMSO as a stabilizer of nanoparticle sizes was confirmed, and the mechanism of primary nanoparticle growth, consisting in their reversible aggregation, was discussed. The aging of colloidal solutions during their long-term storage was also studied.</p>